The present invention relates to gas generant compositions that upon combustion produce a relatively smaller amount of solids and a relatively abundant amount of gas. It is an ongoing challenge to reduce the amount of solids and increase the amount of gas thereby decreasing the filtration requirements for an inflator. As a result, the filter may be either reduced in size or eliminated altogether thereby reducing the weight and/or size of the inflator. Additionally, reduction of combustion solids provides relatively greater amounts of gaseous products per gram or unit of gas generating composition. Accordingly, less gas generant is required when greater mols of gas are produced per gram of gas generant. The result is typically a smaller and less expensive inflator due to reduced manufacturing complexity.
Yet another concern is that the compositions that contain ammonium nitrate, an acceptable oxidizer for low solid or “smokeless” gas generants, must exhibit burn rates that are satisfactory with regard to use in vehicle occupant protection systems. Furthermore, the compositions containing ammonium nitrate must retain their physical integrity. It is known that ammonium nitrate when cycled through various temperature regimes undergoes phase changes that contribute to an unstable propellant tablet that may in fact fracture because of the changes in phase of ammonium nitrate. As a result, ongoing efforts utilize various methods and additives that when combined with ammonium nitrate, attenuate or altogether eliminate the phase changes of ammonium nitrate. Phase stabilized ammonium nitrate (PSAN) incorporating co-precipitated potassium nitrate is one example. The ammonium nitrate must nevertheless be processed with potassium nitrate or some other stabilizing agent thereby resulting in a phase stabilized ammonium nitrate. Even with the use of potassium nitrate to stabilize ammonium nitrate, modest phase changes are still apparent when evaluating ammonium nitrate stabilized with potassium nitrate. To ensure dimensional stability of gas generating pellets containing PSAN, for example, it is therefore an ongoing effort to improve the phase stability of compositions that contain ammonium nitrate.
Thermal stability is yet another concern with compositions containing phase stabilized ammonium nitrate and a primary fuel. When heat-aged at 107C. for about 400 hours, many compositions are not thermally stable due to decomposition. The related performance concerns include ballistic performance and control.